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In Modules/_posixsubprocess.c, (the helper module for Lib/subprocess.py) there's a function called _sanity_check_python_fd_sequence which checks, as its comment says, if the fds in the sequence are all positive and sorted.
The check to verify if it is sorted is incorrect (missing the update to the prev_fd variable), and also it is missing a test to see if fd's are repeated (which they shouldn't be, so the test should use <= rather than <). 
The attached patch, written against Python 3.4.3 source code, fixes it.